Conventionally, for an insulated wire used for wiring of parts for a car such as an automobile, parts for an electrical/electronic appliance, and parts for other devices, there has been widespread use of an insulated wire in which a conductor is covered with a vinyl chloride resin composition to which a halogenous flame retardant is added.
However, this kind of vinyl chloride resin composition includes halogen elements, so that it emits harmful halogenous gas into the atmosphere in case of car fire or at the time of disposing of an electrical/electronic appliance by incineration, causing environmental pollution.
From the view point of reducing loads on the global environment, an olefin resin composition containing an olefin resin such as polyethylene has been recently used as an alternative to the vinyl chloride resin composition. Because the olefin resin is flammable, metal hydroxide such as magnesium hydroxide is added as a flame retardant to the olefin resin composition in order to achieve sufficient flame retardancy.
When this kind of insulated wire is used in a hot environment of an automobile, for example, heat resistance is required. In order to improve heat resistance of an insulated wire, an insulating layer of the insulated wire is often crosslinked.
Examples of crosslinking include electron irradiation crosslinking, chemical crosslinking, and water crosslinking. Electron irradiation crosslinking and chemical crosslinking require expensive, large, specialized crosslinking equipment, which leads to an increase in the cost. Thus, water cross linking which does not have such a problem and allows for easy crosslinking is widely used in recent years.
For example, Japanese Patent Application Unexamined Publication No. 2000-1578 discloses a process of producing a flame-retardant silane-crosslinked polyolefin composition including the steps of forming a compound by kneading an olefin resin with metal hydroxide, a silane coupling agent, a cross-linking agent, a siloxane condensation catalyst, and other elements at once, and heating and molding the compound.
Japanese Patent No. 3457560 discloses a process of producing a flame-retardant silane-crosslinked polyolefin composition including the steps of kneading an A material in which a compound prepared by graft polymerizing a silane coupling agent onto an olefin resin is mixed with metal hydroxide, and a B material prepared by mixing an olefin resin with a cross-linking catalyst and a cross-linking agent, and heating, cross-linking, and molding them.